This invention relates to the comparison of the enthalpies of at least two bodies of air, and more particularly, to a diaphragm module arrangement connected to temperature and humidity sensors located in the two bodies of air for providing an output signal dependent upon these enthalpies.
In typical air conditioning systems, return air which is recirculated from the spaces being supplied with conditioned air is mixed with fresh air drawn from a source of outdoor air and is then filtered, de-humidified and controlled at a temperature sufficient to satisfy the needs of the spaces under control. The circulation of the air in the air conditioning system is controlled by operation of a fan. In modern day air conditioning systems, a so-called "economizer cycle" is used which relies upon outdoor air as a free source of cooling for those spaces requiring cool air. For example, in large office buildings which have both exterior zones or spaces and interior zones or spaces, the interior zones represent a cooling load to the air conditioning system during all months of the year, whereas the exterior zones represent a cooling load during summer months and a heating load during winter months. Therefore, under advantageous conditions, it is more economical to rely upon outdoor air, rather than a refrigeration unit, as a source of cooling for those zones which require cooling.
The function of the economizer cycle is to allow the introduction of outdoor air into the air conditioning system as long as the outdoor air is not too cold or too hot and/or too humid. The economizer cycle, therefore, requires the temperature and moisture content of both the outdoor air and the return air to be measured in order to make the decisions of when and how much outdoor air should be introduced into this system. For example, if the outdoor air is warmer than the return air, more energy would be required to cool the outdoor air than the return air; therefore, less outdoor air should be used. Alternatively, if the outdoor air is more humid than the return air, more energy would be required to dehumidify the outdoor air; therefore, less outdoor air should be used.
In making the decision of when and how much outdoor air should be used, it is preferable to measure the enthalpies of the outdoor air and return air since enthalpy is a function of the specific heat content and the latent heat content of a body of air, specific heat being related to dry bulb temperature and latent heat being related to humidity. The measurement of the enthalpy of a body of air gives the most accurate measurement of the de-humidification and either cooling or heating which would be necessary to control the air at a point to satisfy the requirements of the zones under control of the system.
As discussed in my co-pending application, Ser. No. 743,427, filed Nov. 19, 1976, in order to measure the enthalpy of air, prior art arrangements measure both the temperature and humidity of the outdoor air as well as the temperature and humidity of the return air. Such arrangements rely upon four sensors for providing this measurement of total heat, i.e., enthalpy, and have required the use of complex circuit arrangements in order to produce a reasonable approximation of enthalpy. These sensors have also been used together with computer programs for calculating enthalpy according to the prescribed enthalpy formula. In order to reduce the number of sensors required to measure this total heat or enthalpy, prior art systems have utilized wet bulb temperature sensors which, if one reviews the psychrometric chart, gives a fairly accurate indication of total heat or enthalpy. However, wet bulb temperature sensors are complex and require a constant source of water.
In my above noted application, I have disclosed an enthalpy calculator for calculating the enthalpy of air. However, when the enthalpies of at least two bodies of air are to be compared, the structure necessary to perform the comparison can be greatly simplified.